Process of cleaning soldering flux and/or adhesive tape with terpenet and monobasic ester

ABSTRACT

Disclosed is an industrial cleaning process in which a contaminant comprising soldering flux and/or adhesive tape residue is removed from a substrate contaminated therewith. The process comprises the steps of contacting the substrate with a terpene-based cleaning composition which is substantially free of water-soluble organic material; removing the composition and solubilized contaminant from the substrate by contact with water having a temperature of about 70° F. to about 140° F. for a period of time of no longer than about 10 minutes to provide a substrate having a contamination rating of no greater than about 14 micrograms NaCl equivalent/square inch (MIL-P-28809A), and to thereby form a mixture comprising the composition, contaminant and water. The process also comprises separating the water from said mixture, the separated water being substantially free of water-soluble organic material and having a chemical oxygen demand of no greater than about 1,000 ppm.

FIELD OF THE INVENTION

This invention relates to the removal of deposits of soldering fluxand/or other residues from a substrate which is contaminated therewith.More specifically, this invention relates to the use of terpenes in animproved cleaning process for removing contaminants such as solderingflux and/or adhesive tape residue from a substrate.

The invention will be described initially in connection with itsapplicability to the cleaning of printed circuit boards. However, theinvention has wider applicability, as described hereinbelow.

In the fabrication of printed circuit boards, it is known to usesoldering fluxes in order to ensure the firm, uniform bonding thereto ofsolder which functions to adhere to the boards components thereof. Suchflux must be removed from the finished board. If not removed, depositsof the flux, in even trace or minute amounts, can cause the circuitry ofthe board to fail. An example of a flux that is widely used in thefabrication of printed circuit boards is rosin flux, a material which iswater insoluble.

Various types of materials have been proposed for cleaning deposits ofrosin flux from the surface of a printed circuit board. Traditionally,chlorinated hydrocarbon solvents were used widely. However, thesematerials are not presently favored because of environmental problemsthat are associated therewith. Examples of other cleaning materials forremoving deposits of rosin flux from substrates contaminated therewithinclude: (a) alkaline cleaning compounds such as alkanol amines; (b) amixture of N-methyl-2-pyrrolidine and an alkanol amine (U.S. Pat. No.4,276,186); (c) terpene compounds; and (d) dibasic acid esters (U.S.Pat. No. 4,934,391).

Although there are advantages and disadvantages associated with the useof each of the aforementioned type cleaning materials, the use ofterpene compounds in cleaning printed circuit boards of flux deposits isbecoming more and more popular because they possess a combination ofunusually desirable properties. The present invention is directed to animproved process for cleaning printed circuit boards and othersubstrates of deposits of soldering flux and/or other residues withterpene-based cleaning compositions.

REPORTED DEVELOPMENTS

In the cleaning operation, deposits of soldering flux are dissolved bythe water-insoluble, liquid terpene compounds as they are contactedtherewith. A review of the related art reveals that specialconsideration is given to removing from the terpene-treated substratethe dissolved contaminants that adhere thereto for those applicationswhich require that the substrate be highly cleaned. Thus, the effectiveremoval from the substrate of the dissolved soldering flux and theterpene solvent from the substrate in which the flux is dissolved is animportant aspect of the overall cleaning process.

The basic patent on the use of terpenes to clean printed circuit boardsof soldering flux is U.S. Pat. No. 4,640,719, assigned to the sameassignee as the present invention. With respect to the removal from theterpene-treated substrate of dissolved flux and other contaminants, thispatent teaches as follows:

"Because they [terpene compounds] are almost completely insoluble inwater, however, the terpenes cannot be directly flushed away by water.Alternatively and preferably, the terpene compounds are combined withone or more terpene emulsifying surfactants. The addition of suchsurfactants facilitates removal of the terpenes from printed wiringboards by rinsing with water, whereby the terpenes are formed intooil-in-water emulsions." (Column 3, lines 54-62)

The '719 patent discloses further that the terpene-based composition caninclude up to 40 wt. % of the emulsifying surfactant, with a preferredcomposition containing 10 wt. % of surfactant. Such surfactants aretypically water-soluble organic materials.

European patent application bearing publication number 0 354 027 A2discloses cleaning compositions comprising a major amount of selectedcyclic terpenes, including para-menthadienes, and a minor amount ofterpene alcohol for use in cleaning a variety of substrates, includingcircuit boards. This patent discloses that the compositions can includesurfactants and specifically states the following:

"It has been found helpful to add surfactants to solvent compositionswhich are to be used in combination with a water rinse. It has beenfound that the addition of the surfactant to the cyclic terpene mixtureenhances the water rinsability of the solvent composition. Further, ithas been found that in dealing with certain types of soil, the additionof a surfactant to the mixture of cyclic terpenes enhances the abilityof the cyclic terpene mixture to remove the soil from the substrate."(Column 4, line 57 to Column 5, line 1)

This publication discloses further that the surfactants can be used inconcentrations of from 0.1 to 10 wt. %.

Another patent which relates to the removal of soldering flux fromsubstrates such as printed circuit boards is U.S. Pat. No. 4,867,800.The cleaning composition which is the subject of this patent comprises aterpene compound and a dibasic ester which has at least 2 wt. %solubility in water at 25° C. This patent discloses that suchcompositions can be used to achieve improved cleaning relative to theuse of terpene-based compositions that include emulsifying surfactants,which the patentee refers to as an unnecessary, but optional ingredientof his composition. The patent discloses also that the terpene-treatedsurface is preferably rinsed with water, such as deionized water. In anexemplary embodiment disclosed in the patent, the rinse water which wasused in the cleaning process had a temperature of about 150° F.

A common characteristic of each of the processes described in theaforementioned publications is the required or preferred use ofwater-soluble organic materials in combination with the terpeneconstituent of the cleaning composition. While considered essential toachieve high grade cleaning, the presence of such water-soluble organicmaterials leads to problems which are associated with other aspects ofthe overall cleaning process.

One significant problem crops up in connection with disposal of therinse water that is generated during the cleaning process. The amount ofsuch rinse water can be substantial, with some industrial applicationsusing, for example, two to five gallons of rinse water per minute ofoperating time. For various industrial applications, it is consideredideal to simply discharge the rinse water into the local sewage system.However, as a result of the use in the cleaning composition ofwater-soluble materials, such discharge typically runs afoul ofenvironmental standards which require that the chemical oxygen demand(COD) of the discharged water be no greater than some defined value. Forexample, some communities specify that the COD should be no greater thanabout 1,000 ppm for industrial water that is discharged into the localsewage system. There are other communities with even higher standards,for example, CODs of no greater than about 300 ppm. However, in usingcleaning compositions of the type described above, the presence of thewater-soluble materials can result in the rinse water having as much as10,000 ppm or more of COD. This indeed presents a problem for thefabricators of circuit boards inasmuch as it requires the fabricators toadopt additional steps to treat the water in a manner such thatenvironmental standards are met. This, of course, increases the cost ofthe overall manufacturing process.

There are reports (unconfirmed) of the use of surfactant-free,terpene-based compositions in applications for removing solder fluxesfrom various types of substrates. However, it appears that, in suchapplications, the terpene-treated substrate is rinsed withsurfactant-containing water or requires that the substrate be rinsedwith either boiling water or near boiling water. This, of course, isundesirable because of the increased costs associated with the provisionof energy that is needed to heat the significant amounts of water thatare required for the rinsing operation.

In view of exemplary shortcomings of the present state of the artrelated to the use of terpene-based compositions to clean soldering fluxfrom the surfaces of printed circuit boards, as described above, thepresent invention is directed to an improved, more efficient andeconomical process which avoids or significantly alleviates suchshortcomings.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided anindustrial cleaning process in which a contaminant comprising solderingflux and/or adhesive tape residue is removed from a substratecontaminated therewith comprising:

(A) contacting the substrate with a terpene-based cleaning compositionwhich is substantially free of water-soluble organic material for aperiod of time sufficient to solubilize the contaminant;

(B) removing the composition and solubilized contaminant from thesubstrate by contact with water having a temperature of about 70° F. toabout 140° F. for a period of time of no longer than about 10 minutes toprovide a substrate having a contamination rating of no greater thanabout 14 micrograms NaCl equivalent/square inch (MIL-P-28809A), and tothereby form a mixture comprising the composition, contaminant andwater; and

(C) separating the water from said mixture, the separated water beingsubstantially free of water-soluble organic material and having achemical oxygen demand of no greater than about 1,000 ppm.

With respect to Step (A) above, the following procedure is used todetermine the content of water-soluble organic material in aterpene-based composition. Equal volumes of water and the terpene-basedcomposition are combined and allowed to separate into two distinctphases or layers. To the extent that the terpene-based compositioncontains water-soluble organic material, it will migrate to the waterlayer. The COD of the water layer is then determined according tostandard technique. When the water layer has a COD of no greater thanabout 1,000 ppm, the terpene-based composition is considered to be"substantially free of water-soluble organic material".

Thus, in accordance with the present invention, it has been found,contrary to the teachings of the prior art, that it is possible toproduce a highly clean surface by the use of a terpene-based cleaningcomposition which is free of or essentially free of emulsifyingsurfactants and/or other water-soluble organic materials. And suchhighly cleaned surfaces can be produced under practical operatingconditions, for example, at ambient temperature or relatively lowelevated temperature and within times and water contact conditions thatare presently used in industrial applications. And of great advantage isthe fact that the water collected from the cleaning process has asufficiently low COD to enable it to be lawfully discharged to a sewagesystem without purification or other treatment. Alternatively, thecollected water can be recycled for use in the cleaning process, asdescribed hereinbelow.

With respect to the high degree of cleaning that can be achieved by useof the present invention, the present cleaning process can be used toproduce levels of residual contamination that are equal to or far belowthe military requirement of 14 micrograms NaCl equivalent/in²(MIL-P-28809A) (see the aforementioned '719 patent, column 7, lines 18to 32 for additional information concerning the evaluation of the degreeof cleanliness of substrates based on the involved military test). Aspecific example of the degree of cleanliness that can be achieved bypractice of the present invention is set forth in the Example sectionhereinbelow.

DETAILED DESCRIPTION OF THE INVENTION

An important characteristic of the present invention is that it iscapable of being used effectively in an industrial cleaning process, asdistinguished from a cleaning process that is simply carried out on alaboratory scale or a pilot-plant scale. The invention can be practicedin either a batch or continuous cleaning process.

Although it is expected that the invention will be used most widely incleaning printed circuit boards, it has wider applicability in that itcan be used to clean any type of substrate which is contaminated withsoldering flux and/or other types of residue that typically contaminatethe surfaces of printed circuit boards, for example, adhesive taperesidue, waxes and fingerprints. Thus, printed wiring boards and otherarticles which require the use of soldering flux in the making ofmechanical, electromechanical or electronic connections with metal canbe cleaned in accordance with the present invention. In the fabricationof such articles, rosin soldering flux is used widely. However, therealso can be used other types of soldering fluxes, including, forexample, synthetic activated fluxes, organic acid-based fluxes andfluxes contained within soldering paste.

The terpene-based cleaning composition for use in the practice of thepresent invention comprises at least about 60 wt. % terpene, morepreferably at least about 80 wt. % terpene, and most preferably about 85to about 95 wt. % terpene. The composition can comprise about 100 wt. %terpene, but preferably is used in admixture with another constituent asdescribed hereinbelow.

The terpene for use in the practice of the present invention is aterpene compound or a mixture of terpene compounds that aresubstantially water-insoluble and that are capable of dissolving theinvolved contaminants. The use of such terpenes to clean printed circuitboards and related types of articles of contaminants is well known. Forpractice of the present invention, the use of para-menthadienes ispreferred, with dipentenes being more preferred and limonene being mostpreferred. Excellent results have been achieved utilizing a mixture ofterpenes in which about 90 to about 95 wt. % of the mixture is limonene(the major portion being d-limonene), with the balance of the mixturebeing other terpene compounds.

A particularly preferred embodiment of the present invention comprises aterpene-based composition which includes also a monobasic ester, whichis a material that is also substantially water insoluble. Such estersgenerally comprise the reaction product of alkyl alcohols and fattyacids, for example, C₁ to about C₆ alkanols and about C₈ to about C₁₈fatty acids. Preferably C₁ to C₄ alcohols are used. A preferredmonobasic ester is methyl laurate, with isopropyl laurate being morepreferred.

The monobasic ester can comprise up to about 40 wt. % of theterpene-based composition, but preferably comprises up to about 20 wt.%, most preferably about 5 to about 15 wt. % of the composition.Particularly good results having been achieved with the use of about 10wt. % of the ester.

It should be understood that a material different from a monobasic ester(an optional, but preferred ingredient) can also be included in theterpene-based cleaning composition. Such material is desirably one whichmeets the COD standards set forth herein. As mentioned above, theterpene-based cleaning composition is substantially free ofwater-soluble material, that is, according to the procedure describedabove, the COD of the water phase of a two-phase mixture of equalamounts of the terpene-based cleaning composition and water is notgreater than about 1000 ppm.

The contaminated surface should be contacted with the cleaningcomposition for a period of time sufficient to solubilize thecontaminant. The time for effecting solubilization will depend on thetemperature of the cleaning composition and the means by which it isapplied. The temperature of the composition can be like that typicallyused for terpene-based compositions in applications of the present type,for example, room or ambient temperature (about 70° F.) to about 150° F.Exemplary means for applying the cleaning composition are immersion,brushing, and spraying, for example, spraying in air or beneath thesurface of the cleaning composition. It is believed that spraying thecleaning composition onto the substrate will be used most widely. Forbatch operations, typical contact times are about 3 to about 5 minutes.For continuous operations, typical contact times are about 30 seconds toabout 2 minutes

The solubilized contaminant is removed from the terpene-treatedsubstrate by contact with water having a temperature of about 70° F. toabout 140° F. Preferably, the temperature of the water is about 110° F.to about 130° F. Water-contact times of the type generally used in theart can be used, for example, not longer than about 10 minutes.Preferably, the water-contact time does not exceed about 5 minutes. Goodresults have been achieved with a contact time of about two minutes.

Conventional means can be used to apply the water. It is believed that,for most applications, spraying the air-borne substrate will besatisfactory and convenient. A spray pressure of about 30 to about 50psi is exemplary. Other means for applying the water can be used also,for example, ultrasonic water agitation.

After the solubilized contaminants have been removed from the substrate,the substrate is dried. Typically, drying will be accelerated by the useof heat. For example, the substrate can be dried in an oven and/or itcan be dried with forced air, for example, heated to 100° F. or above.

With respect to the degree of substrate cleanliness that can be achievedby the practice of the present invention, levels of contamination thatmeet or are below the military specification MIL-P-28809A requirement of14 micrograms NaCl equivalent/square inch can be achieved. The inventionhas been used effectively to provide substrates having a contaminationlevel below about 10 micrograms NaCl equivalent/square inch.

It should be understood that such low levels of contamination can beachieved without the use of a surfactant in the water which is used toremove the solubilized contaminant from the substrate. Indeed, the CODof such water should be such as to meet the COD requirements set forthherein. Deionized water is used preferably.

The effluent produced by the cleaning process comprises a liquid mixtureof the terpene-based cleaning composition, contaminant dissolvedtherein, and water. The liquid mixture may contain some solids, forexample, minute pieces of solder and miscellaneous debris. The nature ofthe liquid mixture produced in accordance with the present invention issuch that the components thereof can be separated conveniently bygravitational force. Thus, upon being fed to a settling tank, solidsprecipitate from the mixture and the liquid components thereof stratifyinto a top layer comprising the water insoluble or immiscible liquids,that is, terpenes and organic materials dissolved therein, includingcontaminants, and an aqueous bottom layer. The nature of the process issuch that the aqueous layer is substantially free of water-solubleorganic material, that is, it has a COD of no greater than about 1000ppm. Indeed, the process of the present invention can be utilizedeffectively to provide an aqueous phase in which the COD thereof isabout 300 ppm or less, for example, no greater than about 150 ppm. And,this can be accomplished without subjecting the water to any purifyingsteps or dilution.

In separating the different phases of the liquid mixture, the toporganic layer can be simply drawn off. If desired, the aqueous phase canthen be discharged directly into the sewage system, while meeting theCOD demands imposed by the involved governmental agency. If desired, theaqueous phase can be recycled for reuse in the cleaning process. Thiswill generally involve the steps of filtration to separate solids andion exchange to remove undesirable inorganic materials, the latter stepbeing preceded by carbon adsorption (if necessary) to remove organicmaterials that may adversely affect the ion exchange step. Other meansmay be used to purify the water.

Other means can be used to separate the liquid phases of the mixture,for example, centrifugation or by use of appropriate membranes.

The terpene compounds can be treated for recycling or they can bedisposed of in an appropriate way, for example, incineration.

EXAMPLES

This example is illustrative of the practice of the present invention.

EXAMPLE NO. 1

A set of three identical printed circuit boards was fluxed with a rosinsoldering flux (ALPHA 627 flux sold by Alpha Metals Inc. of Jersey City,N.J.), wave soldered, and then cleaned of the flux residue that wasadhered to each of the boards.

The cleaning composition contained terpenes which were derived fromorange peels and which comprised about 95 wt. % limonene and 5 wt. % ofa mixture of other terpene compounds. The cleaning composition wassubstantially free of water-soluble organic materials. The boards wereplaced in the rack of an industrial dishwasher (semi-aqueous cleaningmodule) sold by Electronic Controls Design as Model 6307. The cleaningoperation involved spraying the boards for about 5 minutes with thecleaning composition which was at room temperature (about 75° F.).

The terpene-treated boards were then rinsed with water to remove thecleaning composition and flux dissolved therein. This was accomplishedby subjecting the boards to a 1-minute, 20-second water wash in acontinuous process carried out in an Electrovert H-500 water rinse unitsold by Electrovert of Arlington, Tex. The spray pressure was about 40pounds/square inch. The temperature of the water was about 120° F. Thewater collected from the rinsing operation has a COD well below 1000ppm. Thereafter, each of the cleaned boards was dried with hot air.

The cleanliness of each of the boards was evaluated by use of an Omegameter 600 SMD ionic testing device (sold by Alpha Metals Inc.). Theaverage for the three boards was a contamination reading of only 9.3micrograms NaCl equivalent/square inch.

The next example is a comparative example which is illustrative of aprior art process.

EXAMPLE NO. C-1

The same procedure as described in Example No. 1 above was followedexcept that cleaning was effected with a composition comprising 90 wt. %of the terpene cleaning composition described in Example No. 1 above and10 wt. % of a mixture of emulsifying surfactants. The mixture comprised50 wt. % of an ethoxylated sorbitan ester and 50 wt. % of an ethoxylatedlinear alcohol. The average for the three boards that were cleaned was areading of 9.4 micrograms NaCl equivalent/square inch--essentially thesame as that of the boards of Example No. 1. However, the rinse waterhas a COD in excess of 5,000 ppm.

As mentioned above, monobasic esters can be used effectively in aterpene-based cleaning composition in which they comprise a minor amountof the composition. It has been found also that a composition comprisinga major amount of a monobasic ester can be used effectively in a varietyof different types of cleaning applications which heretofore involvedthe use of halocarbons and alkaline detergents, the use of each of theaforementioned being accompanied by serious disadvantages. The monobasicesters, which can be used to clean a variety of surfaces, for example,metals, plastics, and electronic assemblies, offer the advantages ofwater insolubility, low toxicity, low skin irritation, high flash pointand low odor.

Examples of monobasic esters that can be used in the cleaning operationare methyl, ethyl, and isopropyl esters of medium chain (about C₈ toabout C₁₈) monobasic acids.

The conditions under which the monobasic esters are used are typical ofconditions used for other organic cleaning solvents. They can be used,for example, at a temperature within the range of room temperature toabout 150° F. or higher and under conditions which involve a contacttime of about 1 to about 10 minutes. Conventional means can be used toapply the monobasic esters to the soiled substrates, for example, spray,agitated immersion, and soaking.

Examples of some cleaning applications in which monobasic esters can beused effectively are as follows.

Methyl laurate (100%) was used to clean small metal parts of metal chipsand cutting oils. The parts were dipped in the methyl laurate, blastedwith compressed air to remove excess liquid and soils, and gauged withmeasuring instruments to determine part dimensions. The results weredeemed to be fully satisfactory relative to the use of a conventionalchlorofluorocarbon (CFC-113).

In another example, soiled laboratory uniforms were dry cleaned in acleaning composition comprising 88 wt. % isopropyl myristate, 12 wt. %of a surfactant mixture consisting of Tergitol NP-4 (4 wt. %), TergitolNP-7 (2 wt. %), Macol 2 (2 wt. %), Cyclogol SBDO (2 wt. %), anddipropylene glycol (2 wt. %). Following the wash, the uniforms wererinsed in water. After drying, the uniforms were found to besatisfactorily clean

In another experiment, the methyl ester of capric acid was used to cleana printed wiring assembly which had been soldered using rosin flux. Thecleaning performance was equivalent to that obtained with aterpene/surfactant mixture (BIOACT® EC-7™, Petroferm Inc.).

Thus, monobasic esters are useful as cleaning agents. They are capableof dissolving a variety of common contaminants and offer the advantagesindicated earlier relative to other cleaning agents.

We claim:
 1. An industrial cleaning process in which a contaminantcomprising soldering flux and/or adhesive tape residue is removed from asubstrate contaminated therewith comprising:(A) contactng the substratewith a cleaning composition comprising terpene and monobasic esterwherein said monobasic ester comprises the reaction product of alkylalcohols and fatty acids said terpene being present in an amount of atleast about 60 wt. % of said composition and said monobasic ester beingpresent in an amount of from about 5 wt. % to about 15 wt. % of thecomposition, said composition being substantially free of water-solubleorganic material and said contacting being carried out for a period oftime sufficient to solubilize the contaminant; (B) removing thecomposition and solubilized contaminant from the substrate by contactwith water having a temperature of about 70° F. to about 140° F. for aperiod of time of no longer than about 10 minutes to provide a substratehaving a contamination rating of no greater than about 14 microgramsNaCl equivalent/square inch [(MIL-P-28809A)], and to thereby form amixture comprising the composition, contaminant and water; and (C)separating the water from said mixture, the separated water beingsubstantially free of water-soluble organic material and having achemical oxygen demand of no greater than about 1,000 ppm.
 2. A processaccording to claim 1 wherein the composition and solubized contaminantare removed from the substrate by water spray.
 3. A process according toclaim 1 wherein said contact with water is effected under conditions toprovide a substrate having a cleanliness rating of below about 10micrograms.
 4. A process according to claim 1 wherein said cleaningcomposition comprises at least about 80 wt. % of terpene compound.
 5. Aprocess according to claim 4 wherein the terpene-based compositioncomprises about 85 to about 95 wt. % of terpene compound.
 6. A processaccording to claim 5 wherein the composition comprises about 10 wt. % ofmonobasic ester.
 7. A process according to claim 6 wherein the ester ismethyl laurate or isopropyl laurate.
 8. A process according to claim 1wherein the water is separated from the mixture by gravitational force.9. A process according to claim 8 wherein the separated water isdischarged to a sewage system.
 10. A process according to claim 9wherein the COD of the separated water is no greater than about 300 ppm.11. A process according to claim 10 wherein the COD of the separatedwater is no greater than about 150 ppm.
 12. A process according to claim1 wherein the substrate is contaminated with soldering flux.
 13. Aprocess according to claim 1 wherein the flux is rosin flux.
 14. Aprocess according to claim 1 wherein the temperature of the water isabout 110° F. to about 130° F. and the period of time is no longer thanabout 5 minutes.
 15. A process according to claim 1 wherein saidremoving step comprises contacting the substrate with a spray of waterhaving a pressure of from about 30 to about 50 lbs./square inch.
 16. Aprocess according to claim 15 wherein said cleaning compositioncomprises monobasic ester in an amount of about 10 wt. % of thecomposition.
 17. A process according to claim 1 wherein said monobasicester is methyl laurate or isopropyl laurate.